This invention relates to an improved brush assembly for conducting electrical power from a fixed power system to a rotating ice protection system. The invention is especially adapted for use with an aircraft propeller ice protection system.
The hazards of aircraft flight in atmospheric icing conditions are well known. Through the years, various workers in the art have developed various techniques for removing or preventing ice accumulation encountered during flight. Certain techniques are particularly adapted to protect specific parts of an aircraft. An electrothermal propeller de-icing system is an example of a specialized system.
In an electrothermal propeller de-icing system, electrothermal de-icers are bonded to the inboard leading edge portions of the individual blades of an aircraft propeller. An example of an electrothermal propeller de-icer is presented in U.S. Pat. No. 4,386,749 issued Jun. 7, 1983 to Sweet et al. (the '749 patent). Electrical power is supplied to the individual de-icers through flexible wire harnesses that act as jumpers between each blade and the propeller bulkhead. The harnesses must be flexible since each blade must be able to rotate about its axis in order to effectuate pitch changes. An example of a wire harness is presented in U.S. Pat. No. 5,020,741, issued Jun. 4, 1991, to Ziegler et al. (the '741 patent).
Electrical power is conducted to the propeller ice protection system through a sliding contact comprising a slip ring assembly and a brush assembly. A typical arrangement is presented in U.S. Pat. No. 4,136,295, issued Jan. 23, 1979, to Sweet (the '295 patent). The slip ring assembly is mounted to the back of the propeller bulkhead facing the aircraft engine. According to a very common embodiment, the slip ring assembly includes a metal dish having an annular channel that receives a plurality of conductive slip rings formed from a copper alloy. The rings are potted into the channel with an epoxy compound which also provides the necessary dielectric insulation between the individual rings and the channel. The wire harnesses are electrically connected to the individual rings. In such manner, electrical power is transferred from the slip rings to the individual de-icers.
Electrical power is transferred to the slip rings through a brush assembly. A particular system may have one or more brush assemblies. Each assembly typically includes a housing that slidably receives two or more electrical brushes. The brushes are normally composed of a carbon based matrix. Carbon brushes and various carbon based brush compounds are well known in the brush and commutation art. A spring biases each brush against a slip ring. Some form of electrical power connection is attached to the housing. The power connection connects the brush assembly to the aircraft power system and may take the form of a shell-type connector or a number of a individual studs. Flexible shunts transfer power from the power connection to each brush. The aircraft power system typically includes a timing and switching device that switches power from brush to brush which thereby switches power to the various de-icers in a predetermined timed sequence.
There are various forms of brush assemblies adapted for use with a propeller ice protection system. An example of a commonly used brush assembly is provided in the '295 patent. This assembly is modular and comprises a series of brush modules that are stacked to provide the necessary number of brushes. An assembly formed as a single unit that predates the modular assembly is also described in the '295 patent. Other assemblies formed as a single unit are known in the art. Such assemblies may also incorporate multiple brushes per slip ring. For example, six brushes could be used in a three slip ring system with two brush riding on each ring. Using multiple brushes per slip ring may be advantageous if current levels are too high to use a single brush per ring.
Installation is difficult with all of these assemblies. The brush assembly is typically installed on a mounting bracket that is attached to the front of the engine facing the slip ring assembly. Each brush must be aligned radially with its respective slip ring. This is normally accomplished using shims between the bracket and the brush assembly. The assembly must also be adjusted to insure that each brush is either perpendicular or nearly perpendicular (with a specified small angular deviation) to its respective ring. In addition, the axial distance between the slip rings and the brush assembly must be appropriately adjusted. This adjustment process is greatly complicated when the mounting bracket blocks access to the brush assembly. Installation is particularly difficult with an "inside" mounting arrangement where the brush assembly is disposed between the mounting bracket and the engine drive shaft.
Installation is further complicated because of the manner in which prior brush assemblies are attached to the mounting bracket. Brush assemblies in the art typically have mounting holes through a portion of the housing for receiving a mounting screw. The mounting screw passes through the mounting bracket and housing and engages a nut which is tightened and clamps the housing against the mounting bracket. This arrangement presents an additional complication since one hand must be used to tighten the screw and another hand must be used to hold the nut. Holding the brush assembly in proper adjustment during this procedure can be very difficult, especially with an inside mounting arrangement.
The springs that bias the brushes against the slip rings have also presented problems in prior brush assemblies. The manner in which spring force is applied to the brush can cause the brush to tip in the housing and increase brush wear. Brush wear is also effected by variation in spring force during service. The helical springs used in many brush assemblies have a spring rate that causes brush force to vary as the brush shortens due to wear. For example, a new brush is longer and compresses the spring further within the housing which establishes an initial brush force. As the brush wears the length inside the housing decreases and decompresses the spring causing a gradual decrease in brush force. This variation can increase brush wear since a given brush compound typically has an optimum range of brush force. This problem was solved by one brush assembly in the art that incorporated constant force springs. Each spring was configured as a reeled metal tape that unreeled as the brush was inserted into the housing. The metal tape reeled back onto the spring as the brush length shortened due to wear. This type of spring proved susceptible to mechanical failure and fatigue during service.
Some brush assemblies in the art have a lid that is removably attached to the housing with screws. The springs are sandwiched between the lid and the brushes within the housing. In at least one prior lid design, the springs merely rest against the lid without being fixed relative to the lid. This arrangement renders the lid difficult to install since the springs must be compressed and have a tendency to slip to one side as the lid is installed. According to other known brush assemblies, holes are provided in the lid in which gauge pins are inserted to measure brush wear. The distance the rod travels before contacting a brush indicates an amount of brush wear. However, other known brush assemblies fail to incorporate wear gauge holes.
Another consideration in propeller ice protection systems involves lightning strike protection. A lightning strike in the proximity of the propeller can feed through the power lines and damage the power system. A transient voltage suppressor may be connected across each power line and the system ground in order to prevent such an occurrence. The transient voltage suppressor acts as an open switch during normal system operation and as a closed switch when a high magnitude transient voltage potential appears across the power line and the system ground. The high potential is thereby shunted to ground when a lightning strike occurs. A metal oxide varistor (MOV) is commonly used for this purpose. According to one common embodiment, the transient voltage suppressor is attached to the mounting bracket spaced from the brush assembly. Leads connect the transient voltage suppressor across each power line connection and ground connection on the brush assembly. The leads present an extra impedance in the shunt circuit between the transient voltage suppressor and the brush assembly which reduces the effectiveness of the shunt circuit. Also, connecting the leads to the brush assembly is an extra step and may be inadvertently omitted leaving the power supply system unprotected from lightning strike.
As is apparent from the previous discussion, an improved brush assembly is desired. In particular, a brush assembly having improved accessibility and a simplified mounting arrangement is desired. A brush assembly having minimized brush force variation with long spring life and minimized brush tip is desired. A lid having wear gauge holes and means for efficiently positioning the springs during installation is desired. And finally, a brush assembly having minimized impedance between the transient voltage suppressor and the power system connection and an automatically connected transient voltage suppressor is desired.